Abstract

A study of limestone cores from the Upper Shu’aiba Member (upper Aptian) in an oilfield in northwestern Oman shows examples of a cementation phenomenon that appears to be widespread in microporous carbonate reservoirs and fundamental for understanding their porosity evolution during burial diagenesis. The studied reservoirs have total porosity of 4 to 36%, which consists mainly of “micropores,” defined here as pores not distinctly visible in a petrographic thin section (thus generally <30 microns diameter). In these mud-rich rudstones, floatstones, packstones, wackestones, and mudstones, calcite cement preferentially filled the larger pore spaces, while micropores remain relatively open in the more porous rocks. The present wide range in total porosity results mainly from varying microporosity of mud matrix and grains. Lack of corroded pore boundaries militates against formation of micropores by late dissolution.

Preferential cementation of limestone macropores can be explained by “pore-size-controlled solubility” (PCS). Interfacial energy effects increase the solubility of very small crystals relative to larger crystals, and because only tiny crystals can fit into micropores, the micropores tend to remain uncemented at the same time as larger calcite cement crystals grow within nearby macropores. A model is proposed for evolution of the pore-throat size distribution of microporous limestones during burial cementation whereby PCS results in preferential filling of larger pores as the pore-throat size maximum is shifted progressively lower. Observations from other microporous limestones indicate that the cement-pore-size relationships observed in the Oman cores may be a general characteristic that develops in association with “chalky porosity” during burial cementation.